1/25/2012

Arctic Ocean freshwater bulge detected

UK scientists have detected a huge dome of fresh water that is developing in the western Arctic Ocean.

The bulge is some 8,000 cubic km in size and has risen by about 15cm since 2002.The team thinks it may be the result of strong winds whipping
up a great clockwise current in the northern polar region called the
Beaufort Gyre.
This would force the water together, raising sea surface height, the group tells the journal Nature Geoscience.
"In the western Arctic, the Beaufort Gyre is driven by a
permanent anti-cyclonic wind circulation. It drives the water, forcing
it to pile up in the centre of gyre, and this domes the sea surface,"
explained lead author Dr Katharine Giles from the Centre for Polar Observation and Modelling (CPOM) at University College London.

Arctic summers have seen a decline in both ice extent and thickness

"In our data, we see the trend being biggest in the centre of the gyre and less around the edges," she told BBC News.
Dr Giles and colleagues made their discovery using radar satellites belonging to the European Space Agency (Esa).
These spacecraft can measure sea-surface height even when
there is widespread ice cover because they are adept at picking out the
cracks, or leads, that frequently appear in the frozen floes.
The data (1995-2010) indicates a significant swelling of
water in the Beaufort Gyre, particularly since the early part of the
2000s. The rising trend has been running at 2cm per year.Model prediction
A lot of research from buoys and other in-situ sampling had
already indicated that water in this region of the Arctic had been
freshening.
This fresh water is coming in large part from the rivers running off the Eurasian (Russian) side of the Arctic basin.
Winds and currents have transported this fresh water around
the ocean until it has been pulled into the gyre. The volume currently
held in the circulation probably represents about 10% of all the fresh
water in the Arctic.

Of interest to future observations is what might happen if the
anticyclonic winds, which have been whipping up the bulge, change
behaviour.
"What we seen occurring is precisely what the climate models had predicted," said Dr Giles.
"When you have clockwise rotation - the fresh water is
stored. If the wind goes the other way - and that has happened in the
past - then the fresh water can be pushed to the margins of the Arctic
Ocean.
"If the spin-up starts to spin down, the fresh water could be
released. It could go to the rest of the Arctic Ocean or even leave the
Arctic Ocean."
If the fresh water were to enter the North Atlantic in large
volumes, the concern would be that it might disturb the currents that
have such a great influence on European weather patterns. These
currents draw warm waters up from the tropics, maintaining milder
temperatures in winter than would ordinarily be expected at northern
European latitudes.
The creation of the Beaufort Gyre bulge is not a continuous
development throughout the 15-year data-set, and only becomes a dominant
feature in the latter half of the study period.
This may indicate a change in the relationship between the
wind and the ocean in the Arctic brought about by the recent rapid
decline in sea-ice cover, the CPOM team argues in its Nature Geoscience
paper.
It is possible that the wind is now imparting momentum to the
water in ways that were not possible when the sea-ice was thicker and
more extensive.
"The ice is now much freer to move around," said Dr Giles.

"So, as the wind acts on the ice, it's able to pull the water
around with it. Depending on how ridged the surface of ice is or how
smooth the bottom of the ice is - this will all affect the drag on the
water. If you have more leads, this also might provide more vertical
ice surfaces for the wind to blow against."
One consequence of less sea-ice in the region is the
possibility that winds could now initiate greater mixing of the
different layers in the Arctic Ocean.
Scientists are aware that there is a lot of warm water at depth.
At present, this deep water's energy is unable to influence
the sea-ice because of a buffer of colder, less dense water lying
between it and the floes above.
But if this warm water were made to well up because of
wind-driven changes at the surface, it could further accelerate the loss
of seasonal ice cover.
The CPOM team is now investigating the likelihood of this
happening with Cryosat-2, Esa's first radar satellite dedicated to the
study of the polar regions.
"We now have the means to measure not only the ice thickness
but also to monitor how the ocean under the ice is changing," says Dr
Seymour Laxon, director of CPOM and co-author of the study, "and with
CryoSat-2, we can now do so over the entire Arctic Ocean."